736 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
of the enteron formed by an arch of columnar hypoblast, hy, and a floor of nucleated 
(periblastic) protoplasm, per, the ill-defined ascending interspace or canal, nec, being 
bordered by indifferent cells, and opening by means of the blastopore into the dorsal 
groove above. ‘This dorsal groove is more fully treated of on another page, and it can 
be no other structure than the primitive involution forming the medullary canal in so 
many forms, but in Teleosteans simply appearing as a transient, ancestral reminiscence, 
and, except for this, now obliterated. Certainly its connection with the subsequent 
permanent neural cavity cannot be demonstrated. 
So rapidly does the dorsal groove become effaced that in a large series of sections of 
early stages none indicate this structure favourably; but a reference to OELLACHER’S well- 
known figures (No. 114) sufficiently shows this, the deep groove in fig. iv. 3, Taf. ii, 
being merely indicated in fig. vii. 5, Taf. ii.; while the figs. in Taf. iv., such as fig. iv. 1, 
show no trace of it, nor can the permanent cavity be said to be more than foreshadowed. 
Owing to the rapid and complete obliteration of the medullary groove, the absence of a 
post-anal canal has been generally accepted for Teleosteans, and for this reason BALFourR, 
though adding a query to his cautious statement, concluded that no neurenteric passage 
was “apparently developed” (No. 10, p. 286). Baxrour and Parker (Phil. Trans., 1885, 
ii. p. 865) speak of the neural canal arising in Lepidosteus as a slit-like lumen, and not due, 
as supposed by OrLiacHeER for Teleostei, to an actual absorption of cells. ‘ When first 
formed, it is a very imperfectly defined cavity, and a few cells may be seen passing right 
across from one side of it to the other” (fifth day after impregnation). The connection in 
Teleosteans between the primitive enteron, no other than the gastrula-cavity (see page 
713), and the primitive dorsal groove cannot be questioned if our interpretation of figs. 
9, 21, and 22, Pl. III., be correct, for the continuity of this groove, nec, and the blastopore, 
bp, is very apparent. The formation of a neural canal by a dehiscence of neurochordal 
cells is a secondary process, and the Teleostei therefore form no exception to the condition 
which so widely obtains in other Vertebrata, and which was demonstrated by GAssER in 
birds, by Kowatewsky, Ba.rour, His, and others in Elasmobranchs, by OwssANNIKOW in 
Cyclostomes, and by Gorre and others in Amphibians. 
Medullary Groove-—The permanent neural canal is formed comparatively late in 
osseous fishes, whereas in most vertebrates its appearance as a groove on the dorsum is a 
very early feature in development. Fora short period, soon after the optic vesicles are 
defined, a transient longitudinal indentation passes along the median dorsal line from 
the head to the tail, just as LereBouLLer figures (No. 95, pl. ii. fig. 36). It may be 
regarded as actually reaching to the lip of the blastopore, though the depression is 
so slight, in the extreme posterior region, that it is in some cases indistinguishable. 
In Rana at a certain stage the hind part of the neural groove cannot be made out. 
Spencer, however (No. 151, p. 97), found that it extends quite to the caudal margin, 
but in this latter region it is obliterated—the cavity closes up, and the nervous cord 
becomes solid. The hind end of the trunk in the embryonic Teleostean often appears 
like a flattened plate, in which the neurochord spreads out like a spatula (PI. III. fig. 16). 
